This invention relates generally to a trigger system for a controlled rectifier and more specifically to a trigger system for a controlled rectifier which is adapted to regenerate a start and stop signal for each phase in response to a start and stop signal for all phases.
The system of the present invention will be described specifically in conjunction with a system for electochemical painting. However, it is to be understood that the system of the present invention could be used in any environment wherein a load is supplied from a single or multiphase source of electrical energy and the start and stop signal for supplying that energy to the load is generated asynchronously with the line voltage. For example, the system could be utilized in electro refining copper wherein the potential on the workpiece must be reversed periodically to prevent fouling. In this situation, interrupting the triggering pulses on one set of control rectifiers and starting triggering on another set to give reversed output potential across the load is utilized to prevent the fouling. A short time later, the procedure is again reversed. This alternate interruption and starting must be balanced to insure balancing of the load through the supply transformer.
In electochemical painting systems, the painting load is supplied, typically, from a source of three-phase potential through a supplying transformer. The secondary of the transformer is connected to the load through a plurality of six controlled rectifiers, three sets of two of the rectifiers being connected in back-to-back relation in the secondary circuit of each phase of the three-phase supply. The start and stop signal for the overall system is either generated automatically or manually wherein a single start and stop signal is generated for the entire system. For example, an automatic system would include a control circuit which would generate an off time control signal and an on time control signal to time the off time and on time for the electrochemical painting process. The off time and on time control circuit could be a resistor-capacitor timing circuit, a pulse counting circuit, or oven a manual timing circuit.
In most of these systems, the off time and on time control circuits are not synchronized with the line, or the control may be synchronized with one phase of the source of supply. In either event, the on time and off time control signals are not generated in synchronism with each phase of the multiphase supply. For example, in the case of the off time and on time control signal being generated in synchronism with one phase, it is seen that the signal may be generated out of synchronism with the other two phases of the supply source. This would create an unbalance for the supply transformer on a phase-to-phase basis wherein one phase may be energized for a greater or lesser period than other phases in the supply. Further, it is possible that an asynchronous start and stop signal may generate a firing signal on one half wave of a particular phase which is greater than or less than the firing signal generated on an opposite half wave. The unbalance creates an unbalance in the supply transformer thereby causing the transformer to become saturated.
This situation is cured by the system of the present invention wherein the start and stop signal which is generated for all phases is stored for each of the three phases and the start and stop signal is modified by being generated in response to the zero voltage crossover point for both the stop and start signal for each of the three phases in the particular example being utilized. Thus, a gating signal for the controlled rectifiers of the three-phase system is modified by the controlled regeneration of that gating signal, the regenerated signals being generated in response to the generation of the start and stop signal and the sensing of the zero crossover point. Thus, the modified gating signals would be three in number and would be out of phase, one with the other, by 120 degrees as exists in the phasing of the three phases.
Accordingly, it is one object of the present invention to provide an improved firing signal for control devices supplying an alternating current source of supply to a load.
It is another object of the present invention to provide an improved firing signal for an alternating source of supply which provides balancing of a supply transformer connected to the alternating source of supply.
It is a further object of the present invention to provide an improved firing signal for a controlled rectifier device controlling the supply of energy from an alternating source to a load wherein the firing of the controlled devices is synchronized with the line voltage.
It is a further object of the present invention to provide an improved firing signal for a controlled device supplying energy from a multiphase source to a load wherein the firing signal is synchronized with the line voltage for each phase of the source.
It is a further object of the present invention to provide an improved firing signal for a control device supplying energy from a supply transformer to a load wherein the firing signal insures nonsaturation of the transformer due to unbalanced firing of the control device.
It is another object of the present invention to provide an improved firing signal for a control device supplying energy from a multiphase source to a load wherein the firing signal is synchronized to provide phase-to-phase balance through the transformer.
It is a further object of the present invention to provide an improved firing system for controlled rectifiers which is simple to assemble, inexpensive to construct and reliable in operation.